This invention relates to a polyoxymethylene multi-copolymer distinguished in injection moldability or, particularly, faster cycle performance and mechanical property.
More specifically, this invention relates to a polyoxymethylene multi-copolymer used for injection molding which is distinguished in faster speed of solidification in the mold in injection molding to allow molding with the molding time reduced and also in the mechanical property on account of the smaller size of spherulites and consequently reduced molding strain.
The invention also relates to a polyoxymethylene resin composition distinguished in surface appearance, crystallinity, mechanical strength and molding efficiency.
The polyoxymethylene resin is known as an engineering plastic well balanced in mechanical strength and impact resistance and is used in extensive fields as automobile parts and electronic equipment parts.
However, the polyoxymethylene resin has a very high crystallinity the spherulite structure growing as the result of which produces cracking or internal tension to reduce the mechanical strength of the resin.
Also, the polyoxymethylene resin is worked up generally by injection molding, and in this field of injection molding, improvement of the productivity through reduction of the molding cycle or the so-called faster cycle is urgently called for.
For improvement of the mechanical strength of the polyoxymethylene resin, it is required to reduce the size of spherulites and homogenize the spherulite structure. For the faster cycle, the method of accelerating the speed of crystallization and thus the speed of solidification in the mold is effective. For implementation of the finer and more homogeneous crystalline structure and improvement of the speed of crystallization, there were proposed various methods of adding nucleating agents comprised of an inorganic compounds, an organic compounds and a polymeric compound. For example, Patent Publications Nos. SH059-129247 and SH055-19942 disclose a method of using a branched or cross-linked polyoxymethylene resin as a nucleating agent, and Patent Publication No. SH048-8254 describes addition of an inorganic compound represented by talc as a nucleating agent.
However, the resin compositions noted in the foregoing Patent Publications Nos. SH059-129247, SH055-9942 and SH048-8254 have not the crystallization speed sufficiently improved, and so not only the satisfactorily faster cycle is not expectable, but the improvement of the mechanical strength through finer and more homogeneous crystalline structure of the obtainable mold is insufficient. Thus, they hardly satisfy the high mechanical strength required for application to electric and electronic equipment parts and automobile parts.